INVESTIGATION OF MICROSTRUCTURE IN POLY[(PARA-HYDROXYBENZOIC ACID)-CO-(ETHYLENE TEREPHTHALATE)] USING NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY

The microstructure and dynamics of liquid crystalline copolymers of p-hydroxybenzoic acid (HBA) and ethylene terephthalate (ETP) were investigated by proton and C-13 solid-state NMR spectroscopy. The HBA/ETP 80/20 (80 mol % HBA) copolymer contains poly(HBA)-like crystallites with a mass fraction of 4-7% and a smallest linear dimension of about 80 angstrom. The dominant crystal structures in HBA/ETP 60/40 (60 mol % HBA) have a similar mass fraction but resemble poly(ethylene terephthalate) crystallites. NMR spin-lock relaxation indicates cooperative motion in both copolymers above 130-degrees-C involving small angular fluctuations of the local polymer chain axis about the local nematic director. A minority "rigid" component in HBA/ETP 80/20, which is 22-28% abundant at 150-degrees-C and about 15% abundant at 290-degrees-C, does not exhibit cooperative motion. HBA/ETP 60/40 displays similar behavior. Static C-13 NMR line shapes indicate that aromatic rings exhibit free rotation about the polymer chain axis within the component that exhibits cooperative motion, while aromatic rings exhibit small-angle (approximately 20-degrees) fluctuations about their substituted axes in the rigid component. C-13 MAS NMR shows that the compositions of the rigid components in both copolymers above 130-degrees-C are nearly the same as the bulk compositions. The observed behavior is consistent with the presence of nonperiodic layer crystallites.